Once Upon a Time meets the world of Arthurian Legend in the Season 5 premiere

"Don't. (...) One day you will have the opportunity to remove Excalibur from its stone. And you mustn't. Leave the sword alone."

Says a mysterious cinema usher to a very young Emma Swan. It's Minneapolis 1989. Disney's "The Sword in the Stone" is playing in a theatre. How does this usher know her name? Why does he tell her not to take the sword from the stone? He says if you do something you're not supposed to do, even if you do it for a good reason, bad things will happen. A young terrified Emma watches young Wart pull Excalibur from the stone as our screen fades to black and then to 3 knights galloping across the land: it's King Arthur, Lancelot and another knight! Arthur pulls Excalibur from the stone... an incomplete Excalibur which is missing the tip.

Zoom in on the Dark One's dagger with Emma Swan's name on it, and you realize that dagger looks an awful lot like the missing piece of Excalibur...

Emma appears in the Enchanted Forest (or nearby) realm, bubbling up through some black liquid. With her very own "Dark One guide" in the form of an imaginary Rumplestiltskin to teach her how to harness the Darkness and truly become the Dark One. She resists giving in to the Darkness at first, but desperation about finding Merlin (the only one who can beat the darkness) almost leads her to killing someone, until talked out of it by the surprise appearance of Hook, Regina, Robin, Snow and Charming (who of course went into full "hero mode" the moment she disappeared).

In this Camelot Merlin has been missing for some time, but before he left prophesied that Emma and the others would arrive to find him. And thus Arthur knew exactly when and where to find the gang from Storybrooke upon their arrival (via a magic portal opened using Light and Dark magic and a wand that Merlin left for his Apprentice). He welcomes them and leads them to Camelot.

Six Weeks Later. They're all back in Storybrooke, with no memories of their time in Camelot. And Emma in full "Dark One" mode. She has become the Dark Swan.

Dark Swan Quotes

"You went to Camelot to get the Darkness out of me. And you failed." 

"For what you all did to me, you're about to be punished." 

"Emma, why are you doing this? (Hook)

Because. I'm the Dark One."

'Tis a very ominous opening to season 5 - aptly titled "The Dark Swan" - taking off mere seconds after the season 4 finale. Looks like the episodes will alternate between "now" in Storybrooke with Dark Swan, and flashbacks to their time and Camelot to see how they failed there. So this means we should get plenty of time to find out what this version of King Arthur and his knights are like! :o)

Elements from Arthurian Legend

• Merlin - only mentioned, although... who was that mysterious usher?
• Excalibur (the real one, not the false sword from s03e02)
• King Arthur - dashing, good looking guy... seems a bit superficial though
• Lancelot - not exiled or dead!!! (c.f. episode s02e01) Although who knows exactly when this takes place in the timeline... could be he just hasn't been exiled/killed yet.
• Sir Kay - Arthur's foster brother of legend. He beats Arthur to Excalibur, but when he tries to pull it out, ignoring Arthur's warning of Merlin's prophecy... he gets turned to dust!
• Camelot - Looking like your typical medieval fantasy castle, with turrets and pennants and all.

Other fun (non-Arthurian) characters

• Merida! 

She's looking for a way to retrieve her brothers who have been kidnapped from the united clans who refuse to recognize her as Queen after her father's death. Unfortunately for her she runs into Emma just as she has Rumplestiltskin whispering in her ear and almost has her heart crushed by Emma.

• Zelena, a.k.a. the Wicked Witch of the West

Still hanging around in Storybrooke (not by choice), waiting for her and Robin's child to be born. Gets tricked into opening a cyclone portal which takes them all to Camelot by lifting up Granny's diner.